Electrochromic glass, commonly referred to as smart windows, change their tinting level or opacity when exposed to light, heat or electricity. Electrochromic (EC) windows find use in controlling the amount of daylight and solar heat gain through the windows of buildings and vehicles, and can save substantial amounts of energy. Several different types of EC materials are known. The three primary types are inorganic thin films, organic polymer films, and organic solutions. Smart windows are generally made by having two outside transparent layers between which is a counter-electrode layer and an EC layer, between which is disposed an ion conductor layer. When a low voltage is applied across the outer conductors, ions move from the counter-electrode to the EC layer causing the assembly to change color. An advantage of EC window is that it only requires electricity to change its opacity, but not to maintain a particular shade.
EC glass typically uses metal oxides as electrodes. The metal oxides have the disadvantage of reacting with chemical agents, such as the electrolytes, in the EC layer, and they add significant weight to the EC glass. Thus, alternatives are needed to the use of metal oxides as electrodes. One possibility is to replace the metal oxides with carbon nanotubes.
Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. (see Iijima et al. Nature 363:603 (1993); Bethune et al., Nature 363: 605 (1993) and U.S. Pat. No. 5,424,054). Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale.
Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have unique mechanical and electronic properties. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, number of concentric shells, and chirality of the tube.
U.S. Pat. No. 6,692,663 to Zhou et al. discloses compositions produced by solvent exchange methods where the compositions can be used as electrically conductive film coatings used on the glass of EC windows, and to coat carbon nanotubes thereby improving the electrical conductivity of the nanotubes. U.S. Pat. No. 6,217,843 to Homyonfer et al. disclose a method for producing fullerene-like metal chalcogenides which may be used as a conductor in electrochromic devices. U.S. Pat. No. 6,426,134 to Lavin et al. discloses melt-extruded SWNTs chemically bonded at one end to a polymer, and their use as electrically conducting film.
These methods do not provide transparent, conductive thin films for deposition on a glass or plastic substrate and processes used can be technically challenging and expensive for large-scale applications. Accordingly, the present invention provides methods and processes for the use of carbon nanotubes as electrodes, especially in smart windows.